1. Field of the Invention
The present invention relates to a pixel circuit of an organic light emitting display.
2. Discussion of the Related Art
There are various types of flat panel displays such as liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays (FEDs), organic light emitting displays, which have been put into practical use. With the growth of the multimedia society, the flat panel displays have been in great demand. Among these flat panel displays, the organic light emitting display has rapid response time, low power consumption, and a self-emission structure. Also, the organic light emitting display has a wide viewing angle, thereby displaying excellent moving pictures regardless of the size of the screen or the position of a viewer. Moreover, the organic light emitting display can be manufactured in a low temperature environment by a semiconductor fabrication process, which is a simple manufacturing process. For these reasons, the organic light emitting display has attracted considerable attention as the next generation display.
In general, the organic light emitting display emits light by electrically exciting an organic compound. To display a predetermined image, the organic light emitting display is provided with N×M organic light emitting diodes that are arranged in a matrix format, and may be voltage driven or current driven. The methods for driving the organic light emitting display may include a passive matrix type and an active matrix type that uses a thin film transistor. In the passive matrix type, an anode electrode is at a right angle to a cathode electrode. The anode electrode is selected by a scan signal and the cathode electrode receives a data signal, so that an organic light emitting diode (OLED) emits light in response to the data signal applied between the cathode electrode and the anode electrode. In the active matrix type, the thin film transistor is connected to an ITO (Indium Tin Oxide) electrode and has its gate electrode connected to a capacitor, so that the OLED emits light depending on a voltage stored in the capacitor.
FIG. 1 is a block diagram schematically illustrating an organic light emitting display according to the related art. Referring to FIG. 1, the related art organic light emitting display includes a display panel 110, a scan driver 120, a data driver 130, a controller 140, and a power supply 150. The display panel 110 is provided with a plurality of data lines D1-Dm, scan lines S1-Sn and pixel circuits P11-Pnm. The data lines D1-Dm may be arranged in a first direction, crossing the scan lines S1-Sn arranged in a second direction. The pixel circuits P11-Pnm are disposed at pixel areas that are defined by the data lines D1-Dm and the scan lines S1-Sn. The controller 140 serves to output control signals to the scan driver 120, the data driver 130 and the power supply 150. The power supply 150 serves to output necessary voltages to the scan driver 120, the data driver 130 and the display panel 110 in response to the control signals received from the controller 140. The scan driver 120 serves to output scan signals to the scan lines S1-Sn connected to the scan driver 120 in response to the control signals of the controller 140. Thus, the pixel circuits P11-Pnm of the display panel 110 are selected by the scan signals. The data driver 130 serves to output data signals, which are synchronized with the scan signals, to the data lines D1-Dm connected to the data driver 130 in response to the control signals of the controller 140. Then, the data driver 130 applies the data signals to the corresponding pixel circuits P11-Pnm through the data lines D1-Dm. Thus, the pixel circuits P11-Pnm emit light in response to the data signals, thereby displaying a predetermined image on the display panel 110.
FIG. 2 is a circuit diagram schematically illustrating a pixel circuit of the organic light emitting display according to the related art. Referring to FIG. 2, the pixel circuit includes a switching transistor MS, a capacitor Cgs, a driving transistor MD, and an organic light emitting diode (OLED). The switching transistor MS serves to transmit a data signal from a data line Dm in response to a scan signal from a scan line Sn. The data signal received through the switching transistor MS is stored in the capacitor Cgs. The data signal stored in the capacitor Cgs is used to generate a driving current for the driving transistor MD. Thus, the OLED emits light depending on the driving current. A driving current IOLED flowing into the OLED may be expressed by the following Equation 1.
                              I          OLED                =                              1            2                    ⁢                                    K              ⁡                              (                                  Vgs                  -                  Vth                                )                                      2                                              [                  Equation          ⁢                                          ⁢          1                ]            Wherein, Vgs denotes a source-gate voltage of the driving transistor MD, and Vth denotes a threshold voltage of the driving transistor MD.
The organic light emitting display having the pixel circuit may be an active matrix type, and may control a luminance by the current IOLED flowing into the OLED. Accordingly, uniformity of characteristics of thin film transistors, particularly, uniformity of the threshold voltages and mobility of thin film transistors should be achieved in order to have a uniform display. The thin film transistor used in the organic light emitting display may be formed using amorphous silicon or low temperature poly-silicon. Since field-effect mobility of the poly-silicon is 100 to 200 times larger than that of the amorphous silicon, the importance of the thin film transistor using the poly-silicon has increased.
However, the poly-silicon may be manufactured by crystallization of the amorphous silicon using an eximer laser to anneal the amorphous silicon. When the amorphous silicon is crystallized, grain size of the poly-silicon may not be uniform due to non-uniformity of the pulse amplitude produced by the eximer laser. As a result, the thin film transistors have different characteristics such that each pixel may have a different brightness at the same gray level.